A scanning electron microscope (SEM) has a higher resolution than an optical microscope in the observation of the surface of an object, and hence it is widely used not only as a device for research but also as an industrial device for the dimension measurement of semiconductor wafer patterns which have increasingly been miniaturized in recent years, the observation of foreign matters on a surface and the like. In the case of the dimension measurement of a semiconductor, a high resolution of several nm at a low acceleration voltage of 1 kV or lower has increasingly been required. The resolution of a SEM depends on how to focus an electron beam into a smaller spot on the surface of a specimen, and hence it is dominated by a diffraction aberration, the chromatic aberration and the spherical aberration of an electron lens as well as the size of the electron source reduced and focused with the lens. The resolution has heretofore been improved by devising an electron optics system, in particular by increasing the reduction ratio of an electron source, optimizing the shape of the object lens by the combination of acceleration and deceleration electric fields, and thus decreasing the aberration.
However, it has already been proved by Scherzer that it is impossible to make the spherical and chromatic aberrations zero with an object lens rotationally symmetrical to the optical axis, and the improvement of a resolution by such conventional measures has been restricted from the aspects of a shape and dimension, machining accuracy, material quality, breakdown voltage and others. In view of the above situation, a method for canceling the aberration of an object lens with a chromatic and spherical aberration corrector made by combining quadrupoles and octupoles is proposed (refer to H. Rose, Optik 33 (1971), pp. 1 to 24), and a SEM having an aberration corrector has been put into practical application by Zach and others in 1995 (refer to J. Zach and M. Haider, Nuclear Instruments and Methods in Physics Research A363 (1995), pp. 316 to 325).
In the event of the actual use of an aberration corrector, the adjustment of the strength of each pole, the alignment of the poles, and the alignment of the whole system including an object lens and the aberration corrector are very important. The above document by Zach and others discloses the method of judging and adjusting the controlled variables of a multipole in consideration of the amounts, directions and symmetry of the blurring of SEM images. Further, the document of S. Uno, K. Honda, N. Nakamura, M. Matsuya, J. Zach Proc. of 8APEM (2004), pp. 46 to 47 and Published Japanese Translation of PCT No. 521801/2003 disclose the method of estimating the magnitude of various kinds of geometric aberrations by deconvolution through the Fourier transformation of plural SEM images and feeding it back to the control of a multipole. Furthermore, Published Japanese Translation of PCT No. 505899/2005 discloses the method of, in the event of astigmatic correction, modulating the beam energy of charged particles, thus obtaining scanning images, and adjusting the alignment of columns from the deviation of the images and the change of the definition. In addition, JP-A No. 355822/2004 discloses the method of applying beam scanning three times to an identical line on a specimen while changing the energy thereof, thus forming three images, and adjusting a chromatic aberration corrector from the deviation among the images and the change of the definition.